This investigation focuses on resolving the biochemical reactions involved in the inactivation and proteolytic degradation of a cell-associated fructosyltransferase (FTase) produced by Streptococcus salivarius. We have concentrated on the FTase inactivation step because this reaction appears to "mark" the enzyme for subsequent protiolysis. Using both intact and permeablized cells, we have found that the inactivation step requires NADH, Cu++ (specifically), and oxygen. It was further shown that the NADH oxidase system is involved in an oxidative modification of FTase that results in a loss of catalytic activity. One or more species of oxygen radicals appear to be involved in the modification-inactivation step.